Add “Next” and “Previous” links in date headers

Add “Next” and “Previous” links in date headers that go to the next or previous tag “sibling” (so When/2018 would link to When/2019 and When/2017).

Additionally, it would be great to lazy-load the next (or previous) sibling, so clicking a link between them wouldn’t even be necessary (credit for this idea goes to mrm himself!).